1. Field of the Invention
The present invention relates to a driving apparatus that drives a liquid crystal light modulator and a light modulating apparatus.
2. Description of the Related Art
A liquid crystal light modulator employing a liquid crystal has been conventionally used. The liquid crystal light modulator is an element that can be used, for example, as variable optical attenuator (VOA). The pulse width modulation (PWM), for example, is known as a modulation system for a drive signal supplied to the liquid crystal light modulator.
The pulse width modulation has an advantage in easy digital control, and in circuit scale and power consumption as compared with a modulation system such as the pulse height modulation (PHM). By supplying a drive signal that controls the pulse width by the pulse width modulation to the liquid crystal light modulator, the light attenuation rate of the liquid crystal light modulator can be adjusted and the light can be modulated. The drive signal and a reference clock for generating the drive signal in the conventional technology are described below.
FIG. 15 is a diagram of an example of a drive signal supplied to the liquid crystal light modulator by a conventional driving apparatus. In FIG. 15, a drive signal in which the pulse width is changed by 32 levels is described. A frame T1 indicates a period during which an effective voltage based on modulation data is applied to the pixel portion of the liquid crystal light modulator. A reference clock 1510 is a reference clock for generating the drive signal. The frame T1 is a period for 32 cycles of the clock signal 1510. The frame T1 is followed by a frame T2. Frames T1 and T2 are set to have the same length.
Drive signals 1520, 1530, and 1540 are the drive signals generated so as to have the pulse width of 8/32 level, 9/32 level, and 10/32 level, respectively. In the pulse width modulation, an arbitrary voltage (effective value) can be achieved by changing, in the frame T1, the ratio (pulse width) between the period during which the voltage is applied and the period during which the voltage is not applied. In the pulse width modulation, the frames are repeated and the polarity, for example, is changed for each frame.
For an accurate and reliable pulse width modulation, a pulse width modulation circuit is usually configured by a digital circuit. The drive signals 1520, 1530, and 1540 are generated by counting the reference clock 1510 according to the modulation data. As indicated by the hatched portions of the drive signals 1520, 1530, and 1540, when the modulation data changes by one, the pulse width changes by one clock width of the reference clock 1510. Accordingly, the frequency of the reference clock requires the drive frequency multiplied by the number of levels or more.
FIG. 16 is a diagram of another example of the drive signal supplied to the liquid crystal light modulator by the conventional driving apparatus. The drive signals shown in FIG. 16 represent the drive signals shown in FIG. 15 with their frequencies octupled. An increase in the pulse width due to a change in the modulation data is reflected on each pulse. The effective voltage applied is the same as that of the drive signal shown in FIG. 15.    [Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2002-162944
In the conventional pulse width modulation, however, when the liquid crystal light modulator employs a liquid crystal element responding at a high speed, a waveform response (flicker) is caused by the liquid crystal element that responds to the pulse of the drive signal, thereby deteriorating the quality of the output light of the liquid crystal light modulator. To solve this, the frame frequency of the drive signal is increased so that the liquid crystal element does not respond to the pulse of the drive signal. However, for a multi-level control, the minimum pulse width of the drive signal must be short. When the minimum pulse width of the drive signal is short, the frequency of the reference clock for generating the drive signal must be high, resulting in an increased power consumption, an increased circuit cost, and a generation of high frequency noise, etc.
To solve the problems of the conventional technology described above, the present invention aims to provide a driving apparatus and a light modulating apparatus that can enhance the quality of the output light of the liquid crystal light modulator.